A conventional focal-plane shutter as disclosed in Japanese Patent Publication No. 21536/1984 is shown in FIGS. 8 and 9. This shutter comprises an opening shutter blade unit and a closing shutter blade unit. The opening shutter blade unit has two shutter blades 101 and 102. The closing shutter blade unit has two shutter blades 103 and 104. Before the shutter release (not shown) of a camera in which this shutter is installed is operated, the shutter opening 105 is covered with these two shutter blade units as shown in FIG. 8 to pervent light coming from the camera lens from leaking toward the surface of the loaded film. When the shutter release is operated, the closing shutter blade unit is once activated and placed above and beside the shutter opening 105 to open it. Subsequently, the opening 105 is opened by the shutter blade 101 of the opening shutter blade unit. Then, the opening 105 is closed by the shutter blade 103 of the closing shutter blade unit.
A mechanism for driving the above-described closing shutter blade unit is shown in FIG. 9, where a closing lever 106 having a first engagement pin 106a engaging with the closing shutter blade unit is rotatably mounted on a support plate 107. The closing lever 106 further includes a second engagement pin 106b. An anchoring lever 108 undergoing the pulling force of a first tension spring 111 is so disposed that the lever can engage the first engagement pin 106a of the closing lever 106. When the anchoring lever 108 is rotated to the left, it disengages from the closing lever 106.
The closing lever 106 has a driving lens 109 which can engage the second engagement pin 106b. A first tension spring 110 is stretched between the driving lever 109 and the closing lever 106. A third tension spring 112 producing a stronger force than the first tension spring 110 is mounted on the driving lever 109.
First, the anchoring lever 108 engages the closing lever 106 to rotate the driving lever 109 to the left. Thus, the driving lever 109 is held in position. Then, the anchoring lever 108 is rotated to the left to disengage the anchoring lever 108 from the closing lever 106. The lever 106 is urged to rotate to the left by the first tension spring 110. The closing shutter blade unit is placed in the aforementioned position to open the shutter opening 105. These members may be driven manually. In recent years, it has been in common practice to drive them with a small-sized motor.
In this conventional focal-plane shutter, the closing shutter blade unit that closes the shutter opening 105 must be once driven so as to open it. For this purpose, the driving lever 109 must be rotated to the left. Prior to this rotation, the anchoring lever 108 is allowed to turn to the left and engage the first engagement pin 106a, in order to prevent the closing lever 106 from interlocking with the driving lever 109 via the first tension spring 110. Thereafter, the driving lever 109 is rotated to the left momentarily against the biasing force of the stronger third tension spring 112 and held in position.
Let P be the force needed to hold the anchoring lever 108 and the driving lever 109 in position. Let Q be the stroke traveled by the levers until they are held in position. The relation of the force P to the stroke Q is shown in FIG. 10. Specifically, the anchoring lever 108 engages the first engagement pin 106a at a point Q.sub.1. Before this point Q.sub.1 is reached, the force P is small, since it is only necessary to release the lever 108. Then, the driving lever 109 is rotated to the left and held in a point Q.sub.2. For this purpose, the lever 109 must be moved to the left momentarily against the strong biasing force of the third tension spring 112 and, therefore, the force P must be made larger.
In this way, the force P increases suddenly between the positions Q.sub.1 and Q.sub.2 during the stroke. Where a small-sized motor is used to drive the levers, if the battery is consumed to such an extent that the peak of the force P between the positions Q.sub.1 and Q.sub.2 is slightly less than the force produced by the motor, then this system cannot cope with this peak. Therefore, it is impossible to drive the driving lever 109 until it is held in position. Where the levers are driven manually, the force P increases suddenly at an intermediate position between the positions Q.sub.1 and Q.sub.2 before the peak is reached. Hence, a queer feeling is given to the operator's hand.